4 Answers
4

“Nous y sommes presque” is correct, and it's idiomatic if the goal is an abstract one, i.e. when the sentence means “we've almost achieved our objective”. It isn't very idiomatic (possible, but not common) to express that a journey is almost over. For that, we'd say “nous sommes presque arrivés”.

“Nous sommes presque là” doesn't work. I can't really explain why; là is a pronoun that means a place that's somewhat distant, and it could fit in context, but the sentence just doesn't sound good. Instead you need to use the pronoun y. Normally y refers to a place that has already been mentioned, but in the sentence “nous y sommes (presque)”, it can stand for an implied goal even if that goal hasn't been mentioned explicitly.

In informal, spoken French, you can use “on y est presque” or “on est presque arrivés”.

The shorter the better, imo! In the contexts where I most often needed to convey this information to my annoying children in the car, even just “Bientôt!” or “Presque!” all by themselves were adequate to respond to their incessant inquiries: “Papa, on arrive bientôt?”/ “Papa, on est arrivés?” (c’est même possible que j’ai répondu des fois avec “Vos gueules” quand j’avais l’impression qu’ils se foutaient de la mienne à l'unisson!) +1
– Papa PouleJan 20 '16 at 23:32

Almost nobody say Nous y sommes presque these days, outside formal contexts. In most cases, "We are almost there" can be translated in spoken French by:

On y est presque. (a goal)

or

On est presque arrivés. (a trip)

Note: Nous is essentially used by foreign people learning French these days. While French people are often eager to correct foreigners trying to speak French, they are never taught about this specific usage as it is not technically a mistake.